1. Field of the Invention
The present invention relates to sampled positioning systems for moving a member between positions, such as is used in a magnetic hard disk drive sector servo system.
2. Description of the Prior Art
A typical positioning system using sampled position signals to which the present invention relates involves the positioning of a data recording head (a position sampling means) over a selected track of a magnetic hard disk drive. Typical contemporary hard disk drive systems provide positioning by two independent modes of operation, i.e., a seek mode and a track-follow mode, both under control of a microprocessor in the disk drive. The seek mode is for moving the head across many tracks to a target track. The track follow mode is typically used to keep the head on one track. A typical disk drive system which uses sampled data systems performs well in the track-follow mode. However, with the same constraints in the seek mode the result is a longer than desired seek time, because low positioning bandwidth is a constraint. The seek mode is necessarily of wider bandwidth due to stability and error constraints of the system. Conversely, the track-follow mode is of narrow bandwidth since the primary objective is to lock the system onto the target track and thereafter minimize run-out due to positioning error, windage, or low frequency disturbances.
The different requirements of the seek mode and track-follow mode mandate two independent positioning modes. One way to achieve this is the use of a separate servo band. Hence the servo data is separate from the recorded data, so the servo and data tracks in a well designed disk drive do track each other, up to a point. This permits disk drive track density as high as 2000 TPI (tracks per inch), with typical disk drive track density around 1300 TPI. Any higher track density causes problems in the mechanical system, i.e. tracking of the recorded data track to the servo track.
It is technically desirable to provide servo position data contiguous with the recorded data. This contiguousness can be accomplished by interspersing servo data with recorded data and thereby invoking a sampled-data approach to position the recording apparatus. Since position data is only available at servo data sampling times, such sampling data systems are necessarily of low bandwidth.
One method to avoid increased bandwidth is to use feed-forward, and provide both a coarse control and a feedback control system to provide a fine control for deviations between the actual and nominal positions. The feedback control system can be low bandwidth, such as in a sampled data system. The feedforward control system provides the approximate input to the actuator (typically a voice coil motor) required to move between positions in time-optimal fashion. The feedforward control system uses an idealized model and precalculates the trajectory, and the model uses all the parameters involved in the systems equations. The feedforward control system therefore puts a constraint on the designer of disk drives. That is, the designer must take into account the characteristics of every part involved, such as the actuator, especially the K.sub.T versus position and K.sub.T versus electric current. K.sub.T is force per unit current; in meter, kilograms, and seconds, units K.sub.T is Newton/ampere; and in English units it is oz./ampere. For torque, K.sub.T is Newton meters/ampere; and in English units K.sub.T is oz. in./ampere. All servo positioning systems depend on the linearity of K.sub.T.